Work on energetics of muscle has shown that length changes during contraction modify contractile activity. The mechanism of this action has been studied by determining how shortening and lengthening change the strength and time course of contraction. Further progress in this work is expected from the application of these principles to a special type of muscle fibers, which are known to be very slow, the tonus fibers of frog muscles. Preliminary experiments have shown that these fibers can after brief stimulation stay in a state of contraction for more than an hour at 1 degree, without being active, and continue to give responses to length changes. This raises questions of a very basic nature, suggesting that the contracted state can be maintained without cycling of the crossbridges and that the strength of contraction can be controlled without the Ca mechanism initiating contraction. Work carried out so far indicates that control of activity follows the same principles in all striated muscles. It can, therefore, be expected that the results apply also to the heart.